Optical testing device and method of testing



Sept25, 1945. c. J. GLASSER I 3 OPTICAL TESTING DEVICE AND METHOD OF TESTING Original Filed March 20, 1941 3 Sheets-Sheet l I Q [illlilili|i|ili liiililllili l Ill llllll INVENTOR. O'zar/asJ 61621561 Sept. 25, 1945. c. J. GLASSER 2,385,503

OPTICAL TESTING DEVICE AND METHOD OF TESTING Original Filed March 20, 1941 5 Sheets-Sheet 2 l'ilililihlilililili 1% aarlesJfiiczsser nul Set, 25, 145 c. .1. GLASfiER 9 Q OPTICAL TESTING DEVEEQE AND METHOD OF TESTING Original Filed March 20, 1941 5 sheets Sheet 3 entering the other cell.

OPTICAL TESTING DEVICE ANDME'IHOD F TESTING Charles J. Glasser, Chicago, Ill.

Original application March 20, 1941, Serial No. 384,372; Divided and this application March 1'1, 1942-, Serial No. 435,119

13 Claims. (am-1.5)

This application is a division of my co-pending application, Serial No. 384,372, filed March 20, 1941, issued December 15, 1942, as Patent No. 2,304,814.

This invention relates to a method of and means for testing lenses to determine their focal characteristics, and is applicable to individual lenses or combinations of lenses which may be placed in the test apparatus, as well as to such lenses as cannot be placed in the test apparatus, for instance, the lens of the human eye.

In the testing of lenses as, for instance, to determine the focal length of the lens, in accordance with the principles of one embodiment of the present invention, light isdirected to a lens under test, and then passed-to two photo-electric cells so arranged that one cell receives all of the light On one side of the optical axis of the lens and the other cell receives all of the light on the other side of the optical axis of the lens. The current or potential of those cells is sent or applied to; a balancing circuit, the output of which is a function of the difference between the outputs 'of the two cells (suitable amplifying means being provided). A light cut-off'is provided for cutting off the light entering one of the cells, without substantially afiectingthe light This cut-ofi is shifted longitudinally along the optical axis of the lens. So long as the c'ut-ofi does not pass the point where the light rays cross the optical axis of the lensthe movement of the cut-ofi produces no relative change in the two light cells. .One light cell. remains entirely out off and the other light cell is not at all cut off, and therefore the outputs of the cells remain unbalanced. When, however, the cut-off reaches the cross-over point where the light rays cross the optical axis of the lens the cut-01f cuts the'lightofi? from neither cell, whereupon a balanced condition of the cells is reestablished. This balanced condition is broken immediately asthe cut-oil passes thecross-over point. This givesa sharp indication oithe cross-over point of the lens, from which the focal length is determined.

Since the light cut-01f from one or the other of the two photo cells is zero at the cross-over point and rises to 100% value for one or the otherof the cells as the-cut-oflf membermoves only a very' slight distance from the cross-over point, there is thus obtained a sharp cut-ofipoint indicating when the cross-over point of light from the lens is reached; This sharp cut-01f point may be utilized to actuate a relay to mark a graph sheet that moves together with the cut-oi! and thus indicate on the graph sheet the exact point of cut-off. Thus, if a series of lenses are to be tested the test may be performed by an unskilled operator and the necessary conclusions, for which skill is needed, may be drawn by an other person from an.examination of the charts thus made. Furthermore, in filling a prescription for eye glasses the lens manufacturer can furnish a chart of the test made upon the glasses furnished to the patient, and the doctorcan readily ascertain whether or not the glasses furnished the patient are in accordance with the prescription. I

While in the preferred embodiment of the present invention it is desirable to have two light cells arranged in opposition, it is within the purview of the present invention to use only one light cell, by omitting one of the cells. The cell receives approximately half of the light when" It is a further object of the present invention to provide an apparatus and means for testing lenses which is applicable to spherical lenses and also applicable to astigmatic lenses and which can be'used to ascertain the axes of astigmatism and to ascertain the focal lengths on the respective axes.

It is a'stlll further object of the present invention to provide a lens testing apparatus the functioning of which will be independent of the degree of luminosity of the light source used. As a result it is possible to use an ordinarmelectric light source energized from a commercial power system which may be subject to fluctuations and/or surges. The arrangement is such that light from the light source actuates two light responsive cells connected diflerentially. If the luminosity of the light source increases or decreases it will afiect both light cells equally. Hence the system would not be influenced by variations in luminosity of the-source whether the variations are due to changes in the light bulb itself or due to electrical surges. p The attainment of the above and further objects of the present invention will be apparent from the following specification taken in conjunction with the. accompanying drawings formin: a pm he s the position the two li t cells 8 In the drawings:

Figure 1 is a diagrammatic view of an optical iystem' embodying the present invention;

Figure 2 is a horizontal sectional view, par tially diagrammatic, of a structure operating on the principles of Figure 1;

Figure 3 is a fragmentary sectional view taken along the line 3-3 of Figure 2;

Figure 4 is an end view of Figure 3;

Figure 5 is a diagrammatic view illustrating another embodiment of the present invention;

Figure 6 is a diagrammatic horizontal sectional view of a structure operating on the principles illustrated in Figure 5;

Figure '1 is a diagrammatic view illustrating still another embodiment of the present invention;

Figure 8 is a horizontal sectional view through a testing apparatus for testing the lens of the human eye;

Figure 9 is a diagrammatic view illustrating another embodiment of the present invention; and

Figure 10 is a diagrammatic fragmentary view showing another modification applicable to the other embodiments illustrated.-

Reference may now be had more particularly to Figure 1. In this figure a light source, indicated at'l, directs light through a narrow slit or aperture 2 in a plate 3 to a lens 4 which is to be tested. The lens is mounted at a fixed distance from the slit 2 and directs its light to a reflecting prism 6, one surface I of which reflects light to an electric light responsive cell 8 and the other surface 9 of which reflects light to an electric light responsive cell If). A masking screen or cut-off I2 is provided between the reflecting prism 6 and the test lens 4. The 'cut-off has a knife edge at its lower end which terminates exactly on the optical axis l4 of the test lens 4 or an exceedingly minute distance above the axis H. The slit 2 and the apex between the prism surfaces I and 9 are also on the optical axis H. The

neutralize one another and the output of the amplifier is zero. The cut-off l2 destroys the balance in that the cut-off obstructs the light from one or the other of the two cells. The cutoff is then moved along the optical axis l4. As it moves along the axis it produces no efiect upon the amplifier until the cut-off momentarily crosses through the cross-over point l5. At this instant the balance is reestablished and the amplifier output again drops to zero. If the cut-off moves past the cross-over point ii the balance is immediately again destroyed. The establishment of balance indicates that the cut-off i2 is at the cross-over point 115 of the lens 4. A suitable scale I3 is provided, which is appropriately calibrated, from which the position of the cut-off can be read, thus reading the focal length of the lens 4 directly. The output of the amplifier goes mask I2 is movable between the prism 6 and the lens 4 while maintaining its lower knife edge always on the optical axis 14. When the cutoff is in the position illustrated in full lines in Figure 1 it cuts off substantially all of the light from the lens which would otherwise strike the prism surface I but does not affect the light from the lens which strikes the prism surface 9. This condition prevails unaltered so long as the cut-' off It moves between the prism 6 and the point l5, which is the cross over point where light rays from the lens 4 cross the optical axis l4. At the cross over point the knife edge of the cut-off l2, permits light to pass to both prism surfaces 1 and 9. When the cut-off l2 moves to the left of the cross-over point [5 it cuts oif'all the" light from the lens 4 that would otherwise strike the prism surface 9 and cuts ofi none of the light from the lens 4 that strikes the surface 1. Thus, for every position of the'cut-ofl l2, otherthan at [5, the cut-off masks out all of the light from only one of the light cells and at the position IE it masks out the light to neither of the light cells. The position l5, it'is apparent, is determinative of the focal point of the lens 4. v

The output of the light cells 8 and i0 is delivered to abridge or differential type amplifier l8. When the two cells receive equal amounts of light the electrical effects of the two cells in the amplifier neutralize one another.

The apparatus is initially adjusted with the cut-off I2 out of position so that the effects of and In on the amplifier I8 to an electric device 86, which may be a visual or audible signal device, or may be a relay for controlling a recording device, or the like.

If the lens 4 under test is a spherical lens the image of the line slit 2 at the focal point i5 will be a point. Hence the angular position of the cut-off H at the point IE will be immaterial, and the cut-off [2 together withthe prism may be rotated about the axis l4 as a center without affecting the balanced condition of the light cells. Also, the cut-off may be rotated independently of the prism without affecting the balanced condition of the light cells. If the lens is, however, astigmatic, then the image of the source of light 2 at the point IE will be a line, rather than a point, and if the knife edge is rotated with respect to the image, while leaving the prism stationary, the balance will be disturbed. This condition may therefore be utilized to determine whether or not the lensis astigmatic for if the cut-off is rotated at the focal point and the balanced condition is not disturbed then the lens is not astigmatic.

In the description thus far given it was assumed that the lens under test was a spherical lens or, if there is astigmatism, it was assumed that the axis of astigmatism was horizontal, that is, parallel to the knife edge of the cut-off 12. If, however, the axis of astigmatism of the lens 4 happens to be inclined at an unknown angle with respect to the knife edge of the cutoff, then movement of the cut-off along the optical axis of the lens will not produce a change in the condition of balance or unbalanceof the system. This indicates that there is astigmatism is at right angles to the edge of the prism at the intersection of the surfaces 1 and 9. If there is astigmatism in the lens under test a cross section of the beam of light from the lens will be an ellipse whose major and minor axes are at right angles to one another and at an inclination to the horizontal determined by the meridian of astigmatism of the lens. The cutofl l2 intercepts half of the section of the beam. The edge of the prism 6 being at right angles to the'edge of the cut-oi! therefore cuts the remaining half of the beam of light into two quadrants. If the edge of the cut-off l2 happens to It is now necesw of the beam is divided by the prism will be of equal areas, and the light effects on the two surfaces 7 and 9 of the lens will be the same. If, however, the knife edge l2 cuts the ellipse of light along a line other than one of the axes of the ellipse then the division of the non-intercepted half of the beam by the forward edge of the prism 6 will be into two quadrants of unequal areas. By rotating the knife i2 and the prism 6 together about the axis l4 as a center, the relative areas 'of these two quadrants is changed. The areas remain unequal until the knife edge comes into a position parallel with one of the axes of the ellipse of light out thereby. At that time, and only at that time, the areas of the two quadrants of light which strike the surface I and 9 of the prism are equal; The cut-off and the prism are then rotated together about the axis id as a center. During this rotation the light eellsremain unbalanced until the knife edge reaches a position parallel to an axis of astigmatism. -In this position the balance is established. Thus as the light cells and the knfe edge are rotated together, while they are'm'aintained at right angles to one another, there is a change in the condition of balance between the light cells as the knife edge passes through a position parallel to an axis of astigmatism. One axis of astigmatism of the lens is thus ascertained to be parallel to the kn fe edge. The prism is then rotated to bring its apex into parallelism with the knife edge of the cut-off (or the cut-off is rotated to bring it into parallelism with the prism edge). The knife edge may then be moved along the optical ax s l4, without rotating the knife edge, until it reaches the focal point of the lens at that axis. In that position and only in that position is balance reestablished. This thus indicates the meridian of one axis of astigmatism and the focal length in that ax s. The cut-oil and the prism are then rotated through exactly 90 I2 is then again shifted along 4 to ascertain the focus of the and the cut-oil the optical axis l lens in the opposite meridian, at right angles to the first meridian. Thus the focal length and angle of astigmatism of a lens can be found. Reference may now be had more particularly to Figures 2, 3 and 4, illustrating, diagrammatically, a physical structure for carrying out the principles of the system of Figure 1. The structure includes a base 20 having twoupwardly extending parallel, inverted V-shaped tracks 2l-2l upon which a ho der 22 for the cut-oil I 2 is provided. 'The holder 22 is slidable along the tracks 2|2l. It has a circular opening in which is rotatably mounted a ring that carries the cut-oil l2. The cut-oil is in the form of asemi-circular disc having a knife edge at the bottom. The holder 22 embraces a lead screw 25 which threads through the holder and is rotatably supported in suitable bearings not shown. Upon rotation of the lead screw 25 the holder 22 is moved in one direction or the other along the tracks 2|. he lead screw 25 is rotated by'a worm wheel 26 keye thereto in mesh with a worm on a shaft 28. The shaft 28 is turned by hand, as by a knurled wheel 29. The lens 4 to be tested is mounted in a holder 30 which may be of any desired construction The plate 3 which has'the slit 2, is rotatably, mountand which is stationary on-the:base 20.

The reflecting prism and the light cells 8 and Hi and the amplifier I8 are mounted in a casing 35. I

From the description thus far given it is apparent that by operating the shaft 28 the cutoif I 2 can be moved along the optical axis it 'until it reaches the cross over point of light rays from the lens 4. At that point, and only at that point, the cut-off does not disturb the light which passes to both cells, and equilibrium is established.

The various parts are rotated by a rotatable shaft' ili that is turned by hand and carries a worm 6|v in mesh with worm wheels @2-43. The worm wheel 42 drives a shaft M to which is splined a clutch 45 and a clutch 46. The clutch H5 is actuated by a crank on a stud shaft M, and

. the clutch 46 is actuated by a crank on a stud shaft 48. The clutch is adapted to engage or disengage the shaft "i l with a pinion shaftv 5B which is in mesh with a gear 5! on the cutoff carrying'ring 24. The pinion 50 extends the full length of the path of longitudinal travel of the cut-off l2 so that the gear Si is in mesh with the pinion 50 throughout the length of the longitudinal travel of th cut-off. Upon rotation of the pinion 50 the gear 5| is rotated to rotate the cut-off l2 about the optical axis Hi.

The clutch 46 is adapted to engage or disengage the connection between the shaft M and a pinion shaft 54 which is in mesh with a spur gear 55 that rotates the slit plat 3. The slit plate- 3 is longitudinally slidable for reasons which will be more fully explained hereafter, and for that reason the pinion 54 is elongated.

The worm wheel 43 drives a shaft 58 which, through a clutch 59, actuated'by a stud. shaft .60, stablishes driving ,connections with a shaft 6i carrying a spur gear 62 in mesh with another spur gear 63 that rotates the casing 35 to rotate the prism 6 and associated light cells 8 and 10. Thus by controlling the clutches and turning the shaft 40 it is possible to rotate the slit 2 or the,

the linear positions of the cut-off 12. To accomed in a sleeve 32 that is slidable on the tracks 2| of the base 20. The-plate 3 has a ring 33 that fits into the sleeve 32.

plish this the pinion shaft 50 carries a worm 65 that drives a worm wheel 66 which turns a shaft 61. The shaft 61 has a spiralmiter gear thereon in mesh with a spiral miter gear 68 which rotates a shaft 69 that carries a miter gear 10 in mesh with a miter gear H on a shaft 12 which carriesa calibrated disc '13. Thus the disc 13 is rotated together with the cut-off I2. -The cutoff l2 has a maximum rotation of 180. The 'gear ratio is such that the disc 13 rotates twice as fast as does the cut-off l2.

To ascertain the linear position of the cutoff 12 the shaft 28 is provided with a pair of spiral miter gears '|5'|6 (Fig. 3) of different sizes, .loose on-the shaft and arranged so that either one, but not both, may be. keyed to the shaft by a clutch 11. The miter gear 15 drives a gear 18, and the miter 'gear It drives a gear 19 both of which are in mesh with a gear that drives a shaft 8|. ,The shaft 8| through a worm 82 and worm wheel 83drives a shaft 84. The shaft 84 carries" a spur gear 85 in mesh with a gear 81 on a shaft 88. The shaft 88 rotates at a speed one-tenth thatiof'the shaft 84. The shafts 84 and 88 have calibrated'discs-90 and 9| thereon.

The arrangement is such that the disc 9| makes has been turned a suficient number of times to move the cut-ofl H the full length of its rectilinear travel. During that tim the disc 90 makes ten revolutions. "I'he'disc 9| may be calibrated in inches and tenths of an inch. The disc 90 is similarly calibrated so that each inch calibration corresponds to a tenth of an inch of travel of the holder 22. If desired the calibration may be in diopters or any other scale. A greater number of multiples may be obtained by a greater number of gears or ratios thereof.

The ring 24 is constructed to receive the cutoif plate i2 in eitherof two positions at right angles to one another. In one position of the cut-off plate it the knife edge thereof is parallel with the apex of the prism 6. In its alternate position the knife edge of th cut-off is at right angles to the apex of the prism 6. In each position the knife edge does not cross the optical axis 14, but is on that axis, being spaced therefrom by an exceedingly minute amount.

Magnetically operated pens or markers Mi], HM and we are provided for making records on the discs i3, 9% and 96. These markers are energized by a circuit controlled from the amplifier IS. The arrangement is such that whenever the bridge amplifier i8 is unbalanced there is suiillcient current flowing through the coils to maintain the pens or markers out of contact with the record discs 13, 90 and 9!. At the instant that the cut-ofli 12 reaches a positionaof balance the pens make a mark upon the charts. This thus indicates not only the focal length of the lens ance has been obtained. A manually operable switch I03 is provided for opening the marker circuit. This may be opened during the time the cut-ofi is being rotated to bring it into proper position with respect to the axis of astigmatism. At that time-the signal l6 indicates the presence or absence of a balanced condition of the light cells.

It sometimes happens that a lens to be tested has a very short focal length, in which case it would be necessary tov bring the cut-off l2 exceedingly close to the lens 4. Under those conditions the margin of error would be increased.

To overcome this difficulty means is provided for- ,4 :but also the angular position in which a balcalibration is the same as though the light source were a distance from the lens illustrated in Fisure 2 so that the calibrated scales 90 and 9| still read the focal length on that basis.

While in Figures 1 and 2 I have shown a testing system wherein the cut-off moves longitudinally towards the lens, it is to be understood that the cut-off may be stationary and the lens moved towards the cut-off. Likewise, while the focusing system of Figure 1 is one wherein the incident light from the test lens is divergent, this also is not an indispensable part of the present invention, since the incident light may consist of parallel rays, as illustrated in Figure 5. Furthermore, the lens to be tested may consist of a single lens or a group of lenses assembled together. Such an arrangement is illustrated in Figure 5. Insofar as the parts of Figure 5 are the same as those .of Figure 1, similar reference numerals have been used. In this instance the light source I directs its light through a narrow longitudinal slit 2 in a plate 3 to a lens I20 which directs the light rays into a parallel beam l2l directed to the lens system 4, which system includes a series of lenses the focal point of which is to be ascertained. The lenses are mounted in a suitable holder which is mounted in the test apparatus so that the lens system 4' is movable in a direction parallel to the optical axis ll of the system. In this instance the cut-off I2 is immovable in a direction longitudinally of the optical axis M. The light from the light source focuses at 15 which is the cross over point of the pencils of light, and then strikes the reflecting prism 6 as before, from which it is reflected to the light cells 8 and III the outputs of which are directed to a balancing bridge type amplifier l8 a heretofore. The system is balanced with the cut-oil out of position, at which time both light cells receive the full amounts of light. Thereafter the cut-ofl is positioned so that its knife edge reaches the optical axis ll of the system and thus cuts 05 light from only one of the light cells. Thereafter the lens system 4' is moved in a direction parallel to the optical axis sired to change the ratio of drive between the v shaft 8| and the shaft 28.- This isautomatically accomplished in the following manner: The clutch I1 is actuated by a bell crank I I0 pivoted at ill and including a pin H2 riding in a slot H3 in an arm ill carried by the sleev 32. As the sleeve 32 is moved to the right the pin ll! rides in the slot 3 until the sleeve reaches its extreme position at which point the arm I pushes the pin 2 to actuate the clutch 11 to disengage the driving connection between the shaft 28 and the gear I1 and to establish a driving connectionbetween this same shaft and the gear 1 6. The gear It drives the same stud shaft 8| as did the gear 15 but at twice the speed.

Thus while the cut-off I2 is beingadiusted with th lightsourcehalfasforfromthelensthe ll thus moving the focal point l5 of the lens system closer and closer to the cut-off l2. This movement is of no effect on the light cells until the cross over point I; coincides with the knife edge of the cut-ofl l2, that is, until the lens is brought into focus on the edge of the cut-ofl. At

'this point, and only at this point, the balance of the light cell is reestablished. If the lens 4' is moved too far to the right, so that the light is focused to the right of the cut-off l2, the balance of the outputs of the light cells 8-"! is again. destroyed for then the cut-off masks out .the light going to the cell 10 rather than to the cell 8. Thus, only at the cross over point is balance established. The establishment of the balance thus indicates that the lens system is in a position such that it focuses the light H5 at the knife edge of the cut-off. 'The position of the lens system may then be read upon a calibrated scale iii.

If the lens system is astigmatic, the axis of astigmatism must be ascertained as before. The cut-ofl is positioned so that its knife edge is at right angles to the apex of the prism 8, and then the lens system 4' is rotated aboutthe axis ll until a position is found where the light cells are balanced. Then the cut-off is positioned with its knife edge parallel to the apex of the prism and the lens system is moved along the axis l4 and a dispersing prism I50.

until a position of balanc is again reached, as previously explained. I,

Figure 6 illustrates, diagrammatically, a physical embodiment of the system of Figure 5. In this system the means for moving the lens system 4' longitudinally of the optical axis I4 comprises a lens holder I26 in which th lens system 4 is mounted, which lens holder is in turn rotatably mounted in a holder 22' through which a lead screw 25' threads. Upon rotation of the lead screw 25' the holder I26 and the lens system 4' is advanced rectilinearly along the optical axis I 4. The lead screw 25' may be rotated in any desired manner, as by a worm wheel I30 keyed to the lead screw and rotated by a worm I3I on a shaft I32, which shaft is manually rotated.

g Means is provided for rotating the lens system 4' about the optical axis I6 as a center. This means comprises a spur gear I34 which is a part of theholder I 26 and which includes a cylindrical portion I35 that is rotatable in the holder .22. The spur gear I34 meshes with a rotatable pinion shaft I38 that may be rotated in any desired manner as, for instance, by a worm wheel I39 keyed thereto, which is turned by a worm I40 on a hand operated shaft I4I. Means, similarto those illustrated in Figure 2, may be provided for reading the angular position of the lens system 4' as well as the accurate electro-magnetic means of Figure 2 for indicating the rectilinear position of the lens when balance is obtained.

In Figure 6 I have shown no means for rotating the reflecting prism 6 with its associated light cells, nor means for rotating the'cut-oflf [2 or the slit plate 3, about the optical axis I4. This may, optionally, be provided, which structure would be the same as is illustrated in Figure 2. The illustration of this structure has been omitted for the sake of simplicity, since that is already illustrated in Figure 2.

In Figure '1 I have illustrated a system wherein a lens or lens system may be tested to, determine.

the focusing characteristics thereof under lights of different wave lengths. In this instance I have shown a testing system such as illustrated in Figure (although it is to be understoodthat the system may be one such as illustrated in Figure l), wherein light which passes through the slit 2 is obtained through a light source I The light source and the prism are mounted on a stand I5I so that the light source together with the prism may be rotated. Light from the source I strikes the prism and is broken up into a band I52 ranging fromthe violet to the red or, if the be from the ultraviolet to the infrared. The plate 3 cuts ofi all of the light except light of the acteristics and defects in the human eye is illustratedin' Figure 8. The apparatus of Figure 8 enables the testing of the human eye to ascertain not only the focusing characteristics of the eye but also to ascertain the astigmatism, if any, and the meridian or axis of astigmatism. A description will flrstbe given of the construction of the apparatus of Figure 8 with a view of ascertaining the focusing characteristics of an eye being tested. The light source is indicated at I and directs its light through the slit 2 in the opaque plate 3, thence through the lens I20 from which parallel raysgo to a transparent mirror.

I15. The mirror may be a plain glass plate the opposite surfaces of which are parallel to one another and arranged in a plane at 45 to the incident light. The mirror reflects an image-of the slit 2 onto a mirror I18 which is parallel to the mirror I15 so that it directs the image of the slit along the path indicated, to the-human eye I19 being tested. The image is focused on' the fundus of the eye and reflected back to the mirror I18, thence to and through the mirror I15 to a focusing lens I80 of known focal length, from which the light passes to the reflecting prism 5 that directs the light to the light cells 8 and I0 the outputs of which extend to the bridge amplifier I8, in the manner previously explained. The circuits from the light cells are balanced against one another so that when equal quantities of light strike the two cells the electrical eflects produced by the two cells counterbalance one another and the net output of the amplifier is zero. The

cut-off I2 is mounted in a ring 24', similar to the ring 24 of Figure 2, which in turn is rotatably mounted in a holder 22a that rides upon the tracks 2I similar to that illustrated in Figure 2.

.The ring 24' is moved longitudinally by moving the holder 22a by means of a lead screw 25 through a worm wheel 26 actuated by a' worm 21 on a shaft 28 which is turned manually to turn from the fundus of the eye are focused. At this point the-cut-off I2 cuts off none of the light from r the lens. I80 to the prism 8 whereby a balanced condition of the amplifier circuit'is maintained.

.- If the cut-off I2 is moved to the right or to the .55 source emits invisible light also, th range will:

left from the cross over point I5 it will cut off. exactly half of the beam of light, whether the movement of the cut-off is in one direction or the other and regardless of the amount of movement. Thus the condition of balance of the light cells is disturbed. f It is thus apparent that the balparticular wave len th which passes through the slit 2. By rotating the prism I50 it is possible to'bring any one of the colors of the spectrum from the prism opposite the slit 2-whereby the testing, system tests the focusing characteristics of the lens 4' for light or actinic 'rays of that particular wave .length. Any other means for obtaining monochromatic light may be used.

In the previous descriptions I have described apparatus for ascertaining focal characteristics of lenses of the type which can be placed into'the apparatus and which focuslight passing'through the lens. The principles of the present invention are also applicable to reflecting. lenses and to lenses which cannot be placed into the apparatus -as, for instance, the lens or fundus of the human eye. An apparatus for testing the. focal charanced condition is obtalned only at the crossover point I5which is a very critical position so that even very slight deviations of the cut-off l2 from thisposition results in an unbalanced condition.

Means is provided for selectively rotating the slit 2 or the cut-off I2. or the prism 6; This means comprises a hand rotatable shaft 40 driving a worm 4| which drives a worm wheel 42 that turns a shaft ,44'. The shaft v44'. may be con-.

nected to turn the slit plate 3 by a clutch 46 which is actuated by a crank on a manually operated stud shaft 48. The clutch-connects the shaft 44' to a pinion 54 in mesh with a spur gear 55 that turns the plate 3. The shaft 447 is also adapted to be connected by the clutch 45 actuated by the hand turned stud shaft 41 to connect the. shaft 44' to the pinion shaft 50 which is in clutch 59 actuated by the crank on the stud shaft gear 62 that is in mesh with the gear 63 that rotates the casing 35in which is mounted the prism 8, light cells 8- IIl and amplifier I8, all in a manner similar to the means used for producing the rotation of the corresponding parts in a the structure of Figure 2. It is important to note that the cut-off I2 and prism 6 rotate about the optical axis I4 as a center and that the slit 3 rotates about the corresponding optical axis I I as a center.

Means is provided for ascertaining and recording the linear and angular positions of the cut- 60 is provided for connecting the shaft 44' to the off at the time balance is obtained. For a'scer- I plete revolution for an amount of turning of the lead screw 25 necessary to move the cut-off I2 from one extreme position to the other extreme position. The means for recording the angular position of the cut-off comprises 'a worm 200 on the pinion shaft 50, which worm drives a worm wheel 2III that drives a stud shaft .202 that carries a miter gear 203 driving a miter gear 204 which turns a shaft 205 that is coupled with a stud shaft 206 by a pair of miter gears 201-208. The shaft 206' carries a calibrated disc 2"). The gearing arrangement is such that the disc 2) makes one revolution for a number of turns of the pinion shaft'w necessary to turn the cut-off I2 through 360, although the gearing may be stepped upso that-the disc 2! makes one'complete turn for one half turn of the cut-off.

Electra-magnetically controlled 1 recording markers I00 and IIII are provided for making a visual impression upon the calibrated discs I95 and 2). These markers are maintained out of position with the calibrated discsor charts 185-2 I 0 until a balanced condition is obtained in the amplifier, at which time no current flows through the electro-magnetic coils, and the markers make their marks on the chart. This happens only when the cut-off I2 is at the focal point III. The charts thus indicate the position ofuthe cut-off when balanced conditions preva In order to permit visual examination of the paticnt's eye while the focusing characteristics are being obtained, the mirror I18 is provided with a small hole therethrough at 220, and the casing of the apparatus is provided with a similar hole 22 I, through which the examiner may view the patient's eye and through which the patient may look to concentrate his attention upon an object on the outside ofthe casing.

In the description of the apparatus of Fig. 8

if it was assumed that there is no-astigmatic error in the eye being tested. If there is an astigmatic error, and the edge of the cut-off is not in the meridian of astigmatism, it is not possible to obtain the focal'length because when the cutoff is at an angleto the meridian of astigmatism the cells 8-") are balanced'in every position of the cut-oil on the optical axis. It is therefore the optical axis mesh with the spur gear 5I on the cut-off carrying ring 24' to turn the cut-off. Likewise thenecessary to-ascertain theymeridian of astigmatism. Y To accomplish this the knifeedge of the cut-off is positioned at right angles to the apex of the prism and then the two are rotated together about the optical axis I4. The light cells remain unbalanced until the knife edge of the cut-off reaches a position parallel to an axis of astigmatism, at which position the cells become balanced. The knife edge'is then moved through (or the prism 6 is moved through 90) to bring the prism edge and the knife edge both into parallelism with one of the meridians In each of the embodiments of the present in vention above described results are obtained either moving the cut-ofl towards and away from the lens or by moving the lens towards and away from the cut-off. It is possible to obtain.

similar results even though the cut-off and the test lens are immovable axially with respect to one another. This is illustrated diagrammatically in Figure 9. In this figure parts similar to those of Figure 1 have been given the same reference numerals. The test lens 4 is at a fixed distance from the slit 2. The cut-off I2 is located between the lens 4 and the slit 2 and is not movable along the optical axis, although it may be rotatable as in the embodiments previously illustrated. In this instance the cut-off I2 cuts off light on one side of the optical axis I4 of the lens So that an unbalanced condition is obtained at the prism. When the prism 8 is in the position illustrated in Figure 9 no light is received by the light cell II). The prism is movable along the optical axis I4. As the prism moves to the left from the position illustrated in 'Figure 9, neither of the light cells 8III is affected quantitatively by the movement untilthe prism crosses the cross over point I6. When the prism is to the left of the cross over point I5 the light cell I0, which previously was receiving no light, immediately receives the full quantity of light, and the light cell 8, which previously received the full quantity of light, now receives no light. The balance is again disturbed but in the opposite direction. When the apex of the prism G isexactly at the point I4, both light cells receive equal quantities of light. The reestablishment of the balanced-conditions thus indicates that the prism has been moved to the point I5.

Instead of moving the prism 6, the prism may also be stationary and the desired result obtained by introducing a movable lens of known focal length between the prism and the cross over point IS in the system of Figure 9. The lens-is positioned so that its optical axis coincides with I4. .When the lens is in such a position that its focal point does not coincide with the point I5 it will not establish balance of the light cells 8-). When the lens is moved along means 14 until its focalpoint coincides with the point I! it will establish balance. This is illustrated in Figure 10 wherein a movable le'ns 230- is positioned at a distance from the cross over point I! equal to the focal length of the lens 230. The position of the known lens 230 at which balance is'established is thus an indicamum indication of intensity.

tion of the position of the cross unknown lens 4.

In the above description I have spoken of focusing light by a lens for the purpose of testing the lens. The term light is used in its broadest sense-to include any actinic rays which can be focused by the lens, whether or not the rays are visible to the human eye. Thus, in testing the human eye invisible-rays, such as infrared rays, can be used so that dilation of the pupil by the eye is avoided. In testing other lenses one can use infrared rays or ultraviolet rays as may be expedient.

The instrument of the present invention may be used to measure the index of refraction of optical glass. The measurement of the index of refraction of a melt of optical glass, at the present time, generally requires the formation of an optical fiat for this purpose. This is an expensive operation. It is possible to measure the index of point i of the refraction of a melt of optical glass by measuring the focal length of a lens made from that glass,

which lens is of a known thickness and known radius of curvature. The focal length of a lens is determined by three factors, namely, thickness of the lens, the radius of curvature thereof, and the index lens is made of charted.

of refraction of the glass. If a a known or measurable radius of curvature and the thickness thereof is measured if the light slit ismoved From the above description it is apparent that I 4 the test to determine the meridian of astigmatism is independent of the test to determine the focal length. As a result the rotation of the apparatus to determine the meridian of astigmatism is eifective regardless of whether or not the apparatus is reached, or vice versa, no indication will be given --from which one of the two unknown factors can be ascertained. Since the two unknown factors, namely, the meridian of astigmatism and the focal length are independent of one another, it is exceedingly diflicult to strike both the meridian or astigmatismand the focal length at the same time unless one knows when one of the unknown factors has been ascertainedso that the search can thereafter be factor. This is possible in accordance with the present invention. It is also apparent, from the above decription, that the change froma condition of balance to a condition of unbalance',.in the search for the meridian of astigmatism, and the change from a condition of balance in the search for the focal length is a sharp change that takes'place critically as the apparatus is moved past the critical. points. Thus it is not necessary for the operator to judge the position of the apex of a smooth curve or to judge the position of maxia signal of gradually varying while in the present description I have spoken of rotating the apparatus with respect to the light beam, or rotating the lens to rotate the light beam, it is within the purview of the present inofthe focal length of the lens.

restricted to a search for the other beam.

another along the change the'amount'of light directed to the light.

ian of astigmatism of vention to use other means for For instance, a suitable prism or prisms may be arranged in the light beam, which prisms may be rotated to rotate the light beam from the lens 4 with respect to the to the cut-off l2. 1

In Figure l'the cut-oil I2 is a control member for controlling the illumination or lack of illumination upon the surfaces 1 and 9 of the prism 6. In Figure 5 the lens 4' is itself the control member for controlling the luminosity upon the different faces of the prism. In the modification illustrated in Figure 9 the prism itself is a control member for controlling the luminosity that is reflected from the surfaces 1 and 9 thereof to the light cells. In the modification illustrated in Figure 10 the known lens 230 is a control member for controlling the luminosity reflected from the surfaces of the prism 6 to the light cells. In each instance the control meinber is moved to a critical position to bring the control member and the cross-over point is of light rays which are converged by the test lens into relationship determined by the focal length of the lens. Also, the control member may comprise the plate 3 in which event the cut-ofi I2 is not movable along the optical axis it. Since the focal length of the lens determines the relative relationship of the crossover point! 5 with respect to the distance between the lens and the light slit 2, it is apparent that towards or away from the lens 4 the cross-over point I5 is shifted. If now the lens 4 is at a fixed distance from the cut oil I 2 and the light slit 2 is moved toward the lens until the cross-over point l5 coincides with the knife edge of the cut-01f l2, the distance between the lens 4 and the light source 2 will be a function The focal length may thus be calibrated or charted in terms of the distance between the light slit 2 and the lens 4.

In compliance with the requirements of the patent statutes I have here shown and described a. few preferred embodiments of my invention. It is, however, to be understood that, the invention is not limited to the precise constructions here shown, the same being merely illustrative of the principles of the invention. What I consider new and desire to secure by Letters Patent is:

1. Apparatus for testing the focusing characteristics of the human eye. said apparatus comof the eye, .light responsive means, light control means controlling the light on only one side of the optical axis of the beam of light, movable means for moving the cross-over point and the light control means with respect to one optical axis of thesystem to responsive means as the control means and the cross-over point come to a predetermined position with respect to one another, recording apparatus including two relatively .movable cooperating record making member gearing connecting one of said members for movement with said movable means, and .me

2. Testing equipment for determining a meridthe human eye, said equips rotating the light prism 6 or with respect controlled by the light ment comprising means forming an optical system including the eye whose meridian o! astigmatism is to be determined, said system including means for directing light to and from the eye so that the cross section of the light beam is al-- tered by the eye in accordance with the astigmatism thereof, means dividing oil a quadrant of the light beam by lines of division at right angles to one another and both lines extending through the optical axis of the system, movable'means tor rotating said dividing means about the optical axis of the system while maintaining the lines of division at right angles to one another, light sen-' sitive means receiving light from said quadrant, recording apparatus including two relatively movable cooperating record making members,

gearing connecting. one of said members for movement with said movable means, and means controlled by the light sensitive means responsive to changes in light received thereby in diilerent angular positions of the quadrant for causing ,cell, movable means for causing the cell to receive light only from diiferent limited portions of the area of the lens depending upon the posi.-. tion of the movable means, to vary the light effect on the cell it the lens is astigmatic, recording apparatus, means gearing a part oi! said recording apparatus for movement with said movable means, and means includ'ed in said recording apparatus and cooperating with said part the recording apparatus and controlled by the light cell for making a record or the position of the movable means at which a variation in the light effect takes place.

4. Means for testing light focusing apparatus comprising means forming an optical system including the light focusing apparatus to be tested, said system including means for directing a beam of light to the apparatus and from the creases trolling the passage of light from the system to the light responsive means, mechanism for rotating the light beam and thelight control means with respect to one another about the optical axis of the system to change the amount or light directed to the light responsive means as the control means and the light beam come into a predetermined angular relationship with respect to one another, recording apparatus, means gearing a record making part of the recording apparatus for operation together with said mechanism, and said recording apparatus including electric actuated means controlled by the light responsive apparatus in response to predetermined changes in the amount of light received thereby and cooperating with. said part of the recording apparatus for controlling a part or the recording apparatus to record the positionof the mechanism at which the changes in the amount 01 light occur.

6. Means for testing an image-forming lens which comprises a light responsive cell, means for directing a beam of light from the lens to the cell, a light control member in the light path, means for moving the control member and the cross-over point or light from the lens with respect to one another along the optical axis 0! light'from the'lens to vary the light eilect on the cell, recording apparatus a record making part of which is geared to the said last named means,-

and means controlled by the light cell and cooperating with said part of the recording apparatus for controlling the recording operation of the recording apparatus.

'7. Apparatus for testing a focusingoptical apparatus which comprises a light responsive device, means lor forming an optical system with the apparatus to be tested as a part thereof, said apparatus to a cross-over point at a location tietermined by the focusing characteristics or the apparatus, light responsive means, light. control means'controlling the passage of light lromthe system to the light responsive means, mechanism for moving the cross-over point and the light control means with respect to one another along the optical axis of the system to change the amount of light directed to the light responsive means as the control means and the cross-over point come into a predetermined position with respect to one another, recording apparatus,

means gearing a record making part of the recording apparatus for movement together with said mechanism. and electric actuated means controlled by the light responsive apparatus in response to the change in the amount or light received thereby and cooperating with said part of the recording apparatus for controlling a part of the recording apparatus-to record the position system including means for directing light to the apparatus and then rrom the apparatus to a cross-over point at a location determined by the focusing characteristics of the apparatus, a control member in the path of the light and controlling the passage oi said light to the light responsive member, means for moving the position of the light control member and the cross-over point or the light with respect to one another along the optical axis 01' the system and produc*- ing a change-in the light directed to the light responsive member as the cross-over point and the control member come into one predetermined position with respect to one another, a recording member, means for moving a part or the recording member in synchronism with the movement crane relative position of the light control member and the cross-over point 01' the light with respect to oneanother, and said recorder including means for making arecord oi the position of the moving part or the recorder at the instant of a change of light received by the light responsive member.

8. Means tor testing an image-forming, lens which comprises a light responsive cell, means f for directing a beam of light from the lens to the of the mechanism at which the change in the t amount of light occurs.

5. Means for testing light'iocus ing apparatus comprising means Iorming an optical system including the light focusing apparatus to be tested. said system including means for directing a beam oilightto the apparatus and trom the apparatus to a cross-over point at aiocation determined by the focusing characteristics of the apparatus, light responsive means, light control means conceil, means for relatively rotating the beam and the cell with respect to one another to vary the light eflect on the cell ii the lens is astigmatic, a light control member in the light path, means for moving the control member and the crossover point of light from the lens with respect to one another along the optical axis 01.1mm; from the lens to vary the light eflect on the cell, recording apparatus a part or which is geared to the said two last named means, and means controlled hy the light cell and cooperating with said part for controlling the recording operation of th recording apparatus.

9. Means for testing light focusing apparatus comprising means forming an optical system including the light focusing apparatus, electrical -light responsive means receiving light from the the light responsive means and cooperating with said part of the recording apparatus for controlling the recording operation of the recording apparatus.

10. Means for testing light focusing apparatus comprising means forming an optical system including the light focusing apparatus to be tested, said system including means for directing a light to the apparatus and from the apparatus to a cross-over point at a location determined by the focusing characteristics of the apparatus, light a responsive means, control means controlling the passage of light from the system to a light responsive means, a recorder, means geared to the recorder for moving the cross-over point and the light control means with respect to one another along the optical axis of the system to change the amount of light directed to the light responsive means as the control means and the cross-over point come into a predetermined position with respect to one another, and recorder controlling means controlled by the light responsive apparatus in response to the change in the amount of light received thereby and cooperating with the part of the recorder that is geared to the movable means for causing the recording action of the recorder.

11. Apparatus for testing a focusing optical device comprising a light cell, means for causing the device to direct a beam of light through a crossover point and then to the cell, a light control .member, movable means for moving the crossover point of the beam and the light control member relatively to one another parallel to the optical axis of the beam to bring the cross-over point of light focused by the device first on one side of the light control member and then on the other side thereof whereby there is a sharp change in the light reaching the light cell when the light control member is at the cross-over point, a recording apparatus, means for moving a part of the recording apparatus in unison with saidmovable means, and means controlled by the light cell and cooperating with said last mentioned part of the recording apparatus for controlling the effective operation of the recording apparatus in response to the sharp change in the light reaching the light cell.

12.- Apparatus for determining optical characteristics of a focusing optical device which comprises a light responsive device, means for directing a beam of light onto said optical device and thence from one side of a plane passing through the axis of said beam of light through a crossover point to the electric light responsive device, a light interceptor in the beam of light extending to a position up to the axis of the beam to interceptall of the light on only one side of said axis, a recorder, meansfor moving said interceptor along the optical axis of the beam so that the interceptor passes through thecrossover point of the beam of light and at the same time moving a part of the recording apparatus an amount proportionate to the movement of the interceptor, and means including a part of the recorder for recording the position of the moving part of the recorder at which there is an abrupt change in the output of the light sensitive device.

13. Apparatus for determining optical characteristics of a focusing optical device which comprises two balanced light cells, means for directinga beam of light to said lens and then from said lens to a cross-over point determined by the focusing characteristics of the lens and thence to each of the two balanced light cells from opposite sides of a plane passing through the axis of the beam, a light interceptor in the beam of light and extending up to the axis of the beam so that the interceptor masks out the light on only one 40 side of said plane to upset the balance of the light cells, means for moving the cross-over point of the beam and the interceptor with respect to one another to bring the cross-over point of the light from the lens to the edge of the interceptor to affect equally the light to both cells to reestablish the balance of the effects of the light cells, a

recorder, means for moving aportion of the recorder in unison with the movement of the crossover point of the light with respect to the interceptor, and means including a part of the recorder for making a record on the moving portion of the recorder of the position thereof corresponding to the position of the cross-over'point and the interceptor at which reestablishment of the balance of the light cells isobtained.

CHARLES J. GLASSER. 

